Gravel vacuum questions

[Myrkk]

As someone else mentioned, I think it depends a lot on the substrate you have. I have two types of sand layered so never wanted to mix them up, vacuuming was light to get rid of unwanted surface detritus. Now I have a layer of leaves on top of the sand in varying states of decomposition so I only lightly vacuum.
In my previous tank I never had issues at water change and had a black line along the front of the tank about one inch down. However with the recent move I have had a couple of issues with vacuuming the sand and fish ill afterwards. This thread is making me realise it could be because the sand is no longer layered but mixed together (I had to remove it to move the tank and it all got mixed), very fine and fine, and could be trapping bacteria that is being released even with a gentle vacuum.

 

[AJ356]

Unless you have an overfeeding issue, gravel vacuum has to be one of the most pointless activities, unless there has been a specific problem that needs tidying or sorting.

 

[Seisage]

Wow, I'm so surprised by this! Although I don't know why I'm surprised, bacteria can multiple like mad after all, but I'm still surprised! Any chance you could share a photo or two of this, pretty please? I need to see it in order to be able to recognise it! Now, if only my sand wasn't black... whoops!

I'm not worried about mine though. The Malaysian trumpet snails are constantly reproducing and turning the sand (and gravel, in by mixed substrate tank) and there are way too many of them because I'm guilty of overfeeding at times too, so snail populations often explode, so even if I didn't have busy little digger cories, the MTS can handle the rest.

I've seen people say that anaerobic pockets are a risk especially if you're disturbing the tank while fish are in there, like rearranging the hardscape or changing out substrate, and that makes more sense to me now. If some rocks or heavy driftwood are compressing the substrate even further in those spots, that would reduce water flow in those areas especially. Cool! Love learning stuff like this, thank you!

I waited to reply to this so I could get you a picture when the sun was actually up haha.
This is what it looks like. I see this all the time in my work. It happens frequently at the public aquarium I volunteer at. I think basically all of our tanks with sand substrate have this, and some of the classes I help teach keep mudflat mud and critters in tanks for the duration of the course, so they also get these black anoxic streaks.

Sorry it's a little hard to see. The lighting in my apartment isn't the greatest, but you can see it as a black smudge against the glass. I think what I'll do during my next water change is gently stir up this spot while holding the siphon hose directly above it, so just in case there's anything unsavory there, it'll get removed very promptly.